Another Pentax Q lens for you (and me...).
This lens is TINY, blurry around the edges, fixed focus, not very bright, but by all accounts great fun, and makes the Q small enough to stick in a pocket.

DOWNSIDE - whilst this is the cheapest (by a fair margin) that I can find, it seems they don't have stock until mid-July which is when it'll get sent out. The next cheapest options (up to the £40-£50 mark) are all imports anyway, so they'll take just as long anyway, and you might even have to deal with customs etc.